In the ranging of an object, that is a measurement of the distance from the observing point to the object, stereo cameras such as CCD (Charge Coupled Devices) image sensor cameras or CMOS image sensor cameras are widely used. However in the use of such stereo cameras, various measurement errors in the system are accumulated to affect the resultant precision of the ranging measurement.
For example, an image distortion (called “distortion”, hereinafter) is one of such measurement errors. The distortion is a deformation of the image taken by a camera as classified as a pin cushion distortion or a barrel distortion. The former is a deformation into a pin cushion shape and the latter a barrel shape both against the rectangular original shape. When the lens system is composed of several lenses, the deformation is not a simple pin cushion distortion or a simple barrel distortion, but a complex of these distortions so that the straight line turns into a wavy line. Such distortion and complex distortion are generated even by a protective glass plate or a protective plastic cover attached to the front position before the lens system.
Using such distorted images, the ranging by the stereo cameras generates errors in the result of computed distance to the object.
It is possible to reduce the distortion not only by the lens composition or the aperture position in the lens system but also by direct correction of the image obtained after image acquisition using the cameras.
A conventional method has been applied to a simply increasing distortion lens system. If the distortion increases in accordance with the distance from the center of the lens system (which is the center of the acquired image), the distortion in the arbitral point on the image can be given in a simple increasing function against the distance r from the center of the image. A simple determination of the distortion is carried out by using a checker pattern which has black and white small squares aligned side by side. The images of the checker pattern charts are taken by a camera against the distance of the location of the checker separated from the camera. Then the distortion observed in the checker pattern through the camera system is given against the distance to the object (which is actually the position of the checker pattern chart). Plural correspondences between the distortion and the distance can generate an approximate function to relate the distortion and the distance. As the results, it is possible to remove the distortion from the image by using reverse computation carried out by such approximate function and the information of the distance to the object (see, for example, reference 1).
However, this distortion removal method can be realized by using the relation between squares on the actual checker pattern and the distorted on-image picture elements which compose the checker pattern. Actually once such relation is memorized in a memory device, it is possible to remove the distortion from the acquired image by using the relation retrieved from the memory device not by using the reverse function.
Reference 1:
Page 3 and the FIG. 5, etc. in JP, H11-355813, A (1999)